Acrylic acid, acrylic acid derivatives, or mixtures thereof are used today in a variety of industrial materials, such as adhesives, binders, coatings, paints, polishes, detergents, flocculants, dispersants, thixotropic agents, sequestrants, and superabsorbent polymers (SAP), which are used in disposable absorbent articles, including diapers and hygienic products. In terms of production process, acrylic acid is typically made today from the two-step catalytic oxidation of propylene, which in turn is produced from fossil resources, such as petroleum or natural gas. More on the oxidation of propylene to make acrylic acid and other production methods can be found in the Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 1, pgs. 342-369 (5th Ed., John Wiley & Sons, Inc., 2004).
Fossil-derived acrylic acid uses resources that are not renewable as it takes hundreds of thousands of years to form naturally and only a short time to consume, and contributes to greenhouse emissions due to its high content of fossil-derived carbon. On the other hand, renewable resources refer to materials that are produced via a natural process at a rate comparable to their rate of consumption (e.g., within a 100-year time frame) and can be replenished naturally or via agricultural techniques. Examples of renewable resources include plants, such as sugar cane, sugar beets, corn, potatoes, citrus fruit, woody plants, lignocellulose, carbohydrate, hemicellulose, cellulosic waste, animals, fish, bacteria, fungi, and forestry products. As fossil resources become increasingly scarce, more expensive, and potentially subject to regulations for CO2 emissions, there exists a growing need for non-fossil-derived acrylic acid, acrylic acid derivatives, or mixtures thereof that can serve as an alternative to fossil-derived acrylic acid, acrylic acid derivatives, or mixtures thereof.
Many attempts have been made over the last 80 years to make non-fossil-derived acrylic acid, acrylic acid derivatives, or mixtures thereof from renewable resources, such as lactic acid (also known as 2-hydroxypropionic acid) and other materials. From these resources, only lactic acid is produced today in high yield and purity from sugar (≥90% of theoretical yield, or equivalently, ≥0.9 g of lactic acid per g of sugar), and with economics which could support producing acrylic acid cost competitively to fossil-derived acrylic acid. As such, lactic acid or lactate presents a real opportunity of serving as a feedstock for bio-based acrylic acid, acrylic acid derivatives, or mixtures thereof.
The overwhelming majority of scientific literature and patent art describe the gas phase dehydration of lactic acid, lactic acid derivatives, or mixtures thereof to acrylic acid, acrylic acid derivatives, or mixtures thereof. However, liquid phase dehydration should offer significant advantages over the gas phase dehydration, for example, lower operating temperature and pressure, longer residence time, lower energy use and CO2 emissions, wide selection of catalyst types (e.g. homogeneous and heterogeneous) and catalysts to choose from, lower coking potential of the catalysts, lower safety concerns, lower potential for lactic acid corrosion, wider selection of reactor designs, etc. U.S. Pat. No. 9,309,180 (assigned to Evonik Industries AG) discloses a process to dehydrate lactic acid and produce acrylic acid in liquid phase with the use of various metal salt catalysts, such as K2HPO4, KH2PO4, BaHPO4, and mixtures of similar salts. At 300° C. and reaction time ranging from 4.4 h to 5.5 h, the yield of acrylic acid was between 0.1 mol % and 1.3 mol %.
Accordingly, there is a need for liquid phase dehydration methods of lactic acid, lactide, or mixtures thereof to acrylic acid with high yield and selectivity.